In recent years, a demand for a communication system capable of transmitting/receiving massive data in high-speed (for example, 40 Gb/s) increases resulting from a rapid popularization of Internet. Besides, a high-gain and broadband amplifier having a band of 40 GHz or more is required as an amplifier used for the communication system as stated above. The amplifiers as stated above are used for a driver circuit driving an LN (LiNbO3) modulator disposed at a front end of a transmission part, a pre-amplifier and an equivalent amplifying portion disposed at a front end of a receiving part, and so on.
Besides, a distributed amplifier can be cited as an amplifier suitable for the use as stated above. Input transistors such as a bipolar junction transistor (BJT) and a field-effect transistor (FET) are included in the distributed amplifier, and the band thereof is determined by a ladder filter constituted by input capacitance of the input transistor and inductance of wiring. Accordingly, the distributed transistor has been used from long ago as an amplifying circuit suitable for the broad band. The distributed amplifier is known as an amplifier capable of bringing out performance of a transistor.
FIG. 11 is a circuit diagram showing a configuration of a conventional distributed amplifier. In the conventional distributed amplifier, n-pieces of unit cells are provided between an input terminal 101 and an output terminal 102. A resistor 105 constituting a terminator is connected to the other end of an input transmission line connected to the input terminal 10, and a resistor 106 constituting another terminator is connected to the other end of an output transmission line connected to the output terminal 102. Input transistors 107-1 to 107-n are connected between the input transmission line and the output transmission line by each unit cell. Gates of the input transistors 107-1 to 107-n are connected to the input transmission line, drains thereof are connected to the output transmission line, and sources thereof are grounded. Besides, two inductors 103 exist on the input transmission line by each unit cell, and two inductors 104 exist on the output transmission line by each unit cell.
A cut-off frequency of the distributed amplifier constituted as stated above is represented by “1/π√{square root over ( )} (LCin)”. Here, “L” is a doubled value of inductance of the respective inductors 103 and 104, and “Cin” is the input capacitance of each input transistor.
However, in this conventional distributed amplifier, a loss of a pseudo transmission line constituted by the inductors 103 constituting the input transmission line and the input capacitances of the input transistors 107-1 to 107-n increases as the input capacitance Cin becomes large, and there is a problem that a frequency characteristic is declining as shown in FIG. 12. Namely, a gain slope appears. In an optical communication, the declining frequency characteristic as stated above (gain slope) incurs deterioration of an output waveform because various frequency components are included in an input signal.
Here, causes why the gain slope appears are described with reference to FIG. 13A and FIG. 13B. FIG. 13A is an equivalent circuit diagram at the input transmission line side of each input transistor, and FIG. 13B is an equivalent circuit diagram at the output transmission line side of each input transistor.
At the input transmission line side (gate side), an attenuation constant αg of the input signal is represented by a next expression (numerical expression 1). Here, “ω” is a frequency of the input signal, “Rg” is a gate resistance of the input transistor, “Z0” is √{square root over ( )}(LC), and “C” is a capacitance subsidiary to the two inductors 103 in each unit cell.αg=ω2Cin2RgZ0/2  [numerical expression 1]
Consequently, the attenuation constant αg depends on the frequency.
On the other hand, at the output transmission line side (drain side), an attenuation constant ad of an output signal is represented by a next expression (numerical expression 2). Here, “Rds” is a resistance between a source and a drain of the input transistor.αd=RdsZ0/2  [numerical expression 2]
Consequently, the attenuation constant ad does not depend on the frequency.
It can be said that the causes of the gain slope exist at the input transmission line from the above. Consequently, it is effective for a suppression of the gain slope to reduce the attenuation constant αg.
Conventionally, a Capacitive Source Degeneration circuit as shown in FIG. 14 is used as a circuit capable of reducing the attenuation constant αg. In this Capacitive Source Degeneration circuit, resistors 109-1 to 109-n and capacitors 110-1 to 110-n are connected in parallel between sources of the respective input transistors 107-1 to 107-n and the ground.
FIG. 15 is a view showing a principle of the Capacitive Source Degeneration circuit. In the Capacitive Source Degeneration circuit, a source voltage of the input transistor is represented by a next expression (numerical expression 3). Here, “Vgs” is a voltage between the gate and the source, and “gm” is a mutual conductance, “Rs” is a resistance value of each of the resistors 109-1 to 109-n, and “Cs” is capacitance of each of the capacitors 110-1 to 110-n. Vs=Vgs(gmRs−jω(CsRsgm−Cin)Rs)  [numerical expression 3]
If “CsRsgm−Cin=0” is achieved, an input admittance Y11 is represented by a next expression (numerical expression 4).Y11=ω2Cin2Rg/(1+gmRs)2+jωCin/(1+gmRs)  [numerical expression 4]
On the other hand, an input admittance Y11′ in the conventional circuit shown in FIG. 10 is represented by a next expression (numerical expression 5).Y′11=ω2Cin2Rg+jωCin  [numerical expression 5]
The numerical expressions 4 and 5 show that it is possible to reduce the input capacitance Cin up to 1+gmRs times when the Capacitive Source Degeneration circuit is used compared to a case when the Capacitive Source Degeneration circuit is not used.
However, when the Capacitive Source Degeneration circuit is used, there is a problem in which variation occurs in the characteristic of the distributed amplifier itself, and a yield may decrease.